1. Field of the Invention
The present invention relates to a display device shaped like a thin film and a sputtering target for producing the same, and more particularly, to a novel display device including an oxide conductive film and an Al alloy film and for use in semiconductor devices, flat panel displays of the active and the passive matrix types such as liquid crystal displays, reflection films, optical components and the like and to a sputtering target for producing the same.
2. Description of the Related Art
Thin film transistors (TFTs) serve as switching elements in a liquid crystal display of the active matrix type for instance, and the liquid crystal display has a TFT array substrate including pixel electrodes and interconnection portions such as scanning lines and signal lines, an opposed substrate which includes a common electrode and is disposed over a predetermined distance facing the TFT array substrate, and a liquid crystal layer which is injected between the TFT array substrate and the opposed substrate. A liquid crystal display of the passive matrix type includes interconnection portions such as scanning lines and signal lines, an opposed substrate which includes a common electrode and is disposed over a predetermined distance facing this interconnection substrate, and a liquid crystal layer which is injected between the interconnection substrate and the opposed substrate. The pixel electrodes may be made of an oxide conductive film such as an indium tin oxide (ITO) film which is obtained by mixing about 10 mass % of tin oxide (SnO) in indium oxide (In2O3).
While pure Al or Al alloy such as Al—Nd is used for the signal lines of the interconnection portions electrically connected with the pixel electrode of such a conductive oxide film (hereinafter also referred as “pixel electrodes”), a multi-layer film made of refractory metal, such as Mo, Cr, Ti and W, is interposed as barrier metal between the signal lines and the pixel electrodes so that the signal lines will not directly contact the pixel electrodes. However, the recent years have seen an attempt for omission of such refractory metal and direct connection of the pixel electrodes with the signal lines.
According to Patent Document 1 (JP, 11-337976, A) for instance, use of pixel electrode of an IZO film obtained by mixing about 10 mass % of zinc oxide in indium oxide realizes direct contact with signal lines.
Patent Document 2 (JP, 11-283934, A) describes a surface treatment method by means of plasma processing, ion implantation or the like of a drain electrode, while Patent Document 3 (JP, 11-284195, A) describes a method of forming, as a gate, a source and a drain electrodes of a first layer, a multi-layer film in which a second phase containing impurities such as N, O, Si, C or the like is stacked. Where these methods are used, clearly, it is possible to maintain the contact resistance with pixel electrodes at a low level even when such refractory metal as that described above is omitted.
The reason of interposing barrier metal according to these conventional techniques is because direct contact between interconnections of Al or Al alloy forming signal lines and pixel electrodes increases the contact resistance and degrades the quality of a displayed image. This is because Al easily gets oxidized and its surface gets oxidized in the atmosphere, and because pixel electrodes which are metal oxides seat in their surfaces a high-resistance Al oxide layer as Al is oxidized by oxygen which is created or added during film deposition. Forming of the insulation layer at the interface of contact between the signal lines and the pixel electrodes increases the contact resistance between the signal lines and the pixel electrodes and deteriorates the quality of a displayed image.
Meanwhile, although barrier metal has a function of preventing oxidation of the surface of Al alloy and keeping an Al alloy film and a pixel electrode in favorable contact, since a barrier metal forming step is indispensable to fabrication of a conventional structure in which barrier metal is interposed at this contact interface, it is necessary to secure a film deposition chamber for forming barrier metal in addition to a film deposition sputtering apparatus for forming a gate electrode, a source electrode and further a drain electrode. Nevertheless, as mass production of liquid crystal panels has realized a low cost, an increase of the manufacturing cost and a drop in productivity due to creation of barrier metal are becoming significant.
Against this background, an electrode material, a manufacturing process and the like for omission of barrier metal are recently demanded. In response, Patent Document 2 proposes addition of one surface treatment step. Meanwhile, permitting continuous film deposition of a gate electrode, a source electrode or a drain electrode within the same film deposition chamber, Patent Document 3 inevitably demands more processing steps. Further, due to different coefficients of thermal expansion between a film which contains impurities and a film which is free from impurities, the phenomenon that a film falls off from a wall surface of the chamber during continuous use is rampant, and therefore, it is necessary to often stop an apparatus for the purpose of maintenance. In addition, since Patent Document 1 requires changing an indium tin oxide (ITO) film which is currently most popular to an indium zinc oxide (IZO) film, the material cost is expensive.
Noting this, the inventors developed the technique described in Patent Document 4 (JP, 2004-214606, A) as a result of extensive research and study in an attempt to establish such a technique with which it is possible to simplify processing steps while omitting such barrier metal as that described above without increasing the number of the processing steps and with which it is possible to obtain excellent electrical characteristics and heat resisting property which realize a low contact resistance at a low electrical resistivity without fail and achieve standardization of the material with a reflection electrode, a TAB connection electrode and the like in a display device.
This technique is an attempt to solve the problem described above by using, as the material of an Al alloy film, Al alloy which contains 0.1 through 6 at % of at least one element selected from group of Au, Ag, Zn, Cu, Ni, Sr, Sm, Ge and Bi and making a part of these alloy components appear as a deposit or concentrated layer at the contact interface mentioned above, and it has been confirmed that among these elements, Al alloy containing a predetermined amount of Ni exhibits excellent capabilities.
By the way, a process temperature for producing a display device recently tends to become low for a better yield and an improved productivity, and for further, there are ongoing endeavors to use a resin having a low heat resisting temperature as a base material. Hence, while a demand for a heat resisting temperature is not very strong, there is a significant demand for an interconnection material having a low electrical resistivity.
The material of source and drain electrodes of amorphous silicon TFTs, one type of display device elements, for instance is required to have a low electrical resistivity and a heat resisting property, and demanded specifications are for example an electrical resistivity of 8 μΩ·cm or lower and a heat resisting temperature of about 350 degrees Celsius. This heat resisting temperature is determined by a maximum temperature applied upon the source and drain electrodes during producing, and this maximum temperature is a temperature of forming an insulation film which is to be formed as a protection film on the electrodes.
It has become possible to obtain a desired insulation film even at a low temperature owing to advanced film deposition techniques, and it is becoming possible to form a protection film in particular on source and drain electrodes at about 250 degrees Celsius. This gives rise to a demand for an interconnection material whose heat resisting temperature is approximately 250 degrees Celsius and whose electrical resistivity is sufficiently low.
Meanwhile, although an Al alloy film generally used from before as an interconnection material for a display device is formed by sputtering, in the case of an Al alloy film formed by this method, alloy components added beyond a solubility limit to Al is compelled to exist in the dissolved state. The electrical resistivity of Al alloy containing an alloy element in the dissolved state is generally higher than that of pure Al. However, when an Al alloy film containing an alloy element beyond a solubility limit is heated, alloy components precipitate at the grain boundaries as an intermetallic compound, and as the Al alloy film is further heated, grain growth advances and Al starts re-crystallizing. While the temperature at which the precipitation and the grain growth occur in this manner is different depending upon the alloy element, the precipitation and the grain growth of the alloy components decrease the electrical resistivity of the Al alloy film.
The compressive stress inside the film increases as the grain growth progresses due to heating, and as the grain growth further progresses due to further heating, the limit will be surpassed and crystal grains will appear to the film surface as hillocks for the sake of stress relaxation. Alloying is effective in holding grains in a halt by means of the intermetallic compound precipitating at the grain boundaries, suppressing hillocks and enhancing the heat resisting property. A conventional approach has been to advance precipitation and grain growth of alloy components utilizing this phenomenon for realization of both a lower electrical resistivity and a heat resisting property of an Al alloy film. However, a lowered process temperature does not encourage sufficient precipitation of conventional alloy components as an intermetallic compound, which leads to a problem that grain growth does not advance and an electrical resistivity does not easily decrease.
For example, although the heat resisting temperature of Al-2 at % Nd disclosed in Patent Document 4 is as high as 350 degrees Celsius or more, the electrical resistivity is only 11.5 μΩ·cm after heat treatment at 250 degrees Celsius for 30 minutes, and although the heat resisting temperature of Al-2 at % Ni-0.6% Nd is as high as 350 degrees Celsius or more, the electrical resistivity decreases down to only 8.7 μΩ·cm after heat treatment at 250 degrees Celsius for 30 minutes, thus still leaving a room for further improvement.